a) Field of the invention:
This invention relates to a condenser optical system used in combination with a light source emitting beams of light with good parallelizing properties such as beam lines of synchrotron radiation (SOR) and X-ray lasers.
b) Description of the prior art:
In general, the value of the refractive index of a substance is close to unity in a soft X-ray region, which fact makes it impossible to utilize a refracting optical system used for visible light and an ordinary reflecting optical system. Hence, a normal-incidence multilayer film optical system, a grazing incidence optical system, or a zone plate is used and in the case of nearly parallel light like SOR in particular, the grazing incidence optical system or the zone plate has been employed for a condenser optical system in the past.
The grazing incidence optical system forms a reflecting mirror by making use of the total reflection of soft X rays where their angles of incidence are near 90.degree. for grazing incidence, that is, to make the rays incident on the surface of the mirror, grazing it. Such grazing incidence optical systems are available in, for example, K-B (Kirkpatrick-Baez) and Wolter types. The K-B type optical system, as shown in FIG. 1, comprises two spherical or cylindrical mirrors arranged at right angles with each other and has the advantage that its manufacture is easy. The Wolter type optical system, as shown in FIG. 2, is constructed from the combination of a hyperboloid of revolution with an ellipsoid of revolution so that the focal points of these aspherical surfaces are in common. This type, which uses the aspherical surfaces, is small in aberration and wide in field of view compared with the K-B type optical system.
On the other hand, the zone plate is an imaging element making use of diffraction and, as depicted in FIG. 3, assumes a flat sheet consisting of the zones of concentric circles in which every other zone is formed to be opaque in respect of soft X-rays. The opaque zones, for which gold absorbing easily X rays is used, are usually made by electron-beam lithographic and holographic methods. The relationship between a radius r.sub.n of the circle of the n-th zone from the center of the zone plate and a focal length f is given by rn.sup.2 =nf.lambda., where .lambda. is the wavelength which has the relationship of f=r1.sup.2 /.lambda..
A microscope utilizing the SOR needs an objective lens of a large NA (numerical aperture) for observation of a sample with high resolution and in this case, the condenser optical system also needs the NA to increase. Further, since it is desirable for the condenser optical system that its convergence efficiency is high, the grazing incidence optical system needs a grazing angle as small as possible in order to increase reflectance with respect to soft X rays. Thus, where various types of optical systems mentioned above make use of the condenser optical system for the SOR, there are problems to be considered.
The K-B type optical system first has the problem that if parallel beams are intended to be brought into a focus, aberration will increase, and suffers the inconvenience that if aberration is intended to be reduced, the NA cannot be increased. Additionally, the size of each mirror surface is inevitably increased and alignment is required for a two-mirror arrangement.
The Wolter type optical system, on the other hand, is very difficult of manufacture due to the facts that a thin straight, cylindrical shape is assumed and a high degree of accuracy of asphericity is required for the ellipsoid and hyperboloid mentioned above.
In the case of the zone plate, the zone width becomes narrow progressively in going from the center to the periphery and when dr.sub.n is taken as the minimum zone width, the numerical aperture is given by NA=.lambda./2 dr.sub.n, but when dr.sub.n =25 nm and .lambda.=10 nm in particular, the numerical aperture will be NA=0.2. Hence, it is extremely difficult to obtain the NA of greater than 0.25.